Auditory and visual average evoked responses (AERs) were studied in families of patients with affective illness and in a small sample of twins with affective illness. In patients with primary affective illness, affective disorder probands and their siblings showed similar AER amplitude/intensity slopes, as did probands and their spouses. Within the probands' families, the "well" and "ill" relatives were not significantly different, with both groups showing significant augmenting (high amplitude/intensity slopes) in comparison with normal groups. This suggests that AER stimulus intensity slope measures and affective disorders are not transmitted by a single genetic factor but that AER may be related to increased genetic vulnerability. Review of the heritability of other electrophysiologic variables also suggested that multiple loci and dominance, epistasis and interactions may be important.